Dies for stamping sheet metal parts are conventionally made of tool steel. Steel is the preferred die material because of its strength and durability, especially for stamping steel sheet. However, such dies are relatively costly due to the extensive machining needed to make them.
Therefore, substantial efforts have been directed toward finding a substitute for steel dies for stamping sheet metal parts. It is well known that epoxy based resins have many of the properties necessary for a good tooling material. For example, they are liquid before cure and can be cast into a mold of a desired shape. The cast dies do not require machining. We were seeking an epoxy tooling composition that could be used to make molds to produce stamped steel body panels for automotive prototype vehicles. We ultimately sought to discover epoxy compositions with properties good enough for use in making production tooling.
One commercial epoxy tooling composition is based on a liquid epoxy resin containing about 85 weight percent ceramic filler. This material is used in the aerospace industry for stamping aluminum aircraft parts. It is knows as Magno Ceram.RTM. and is made by Magnolia Plastics Co. However, when this material was used to stamp conventional automotive body sheet steel which is much more difficult to deform then aluminum, the edges of the tools were severely chipped. Magno Ceram is not readily repairable, so it proved to be unsuited to our needs.
A method of making epoxy tools with relatively high compressive strength involves laying fiberglass mats into a mold having a mold cavity in the shape of the tool. After several layers of mat are built up, they are impregnated with liquid epoxy resin. However, such tools also tend to chip and are not repairable. The structural integrity of such a tool is destroyed once the fiberglass mat is exposed.
Another commercial product that we examined was Ren TDT-177 sold by the Ren Plastics Division of Ciba-Geigy. It is based on a diglycidyl ether of bisphenol-A that is filled with molybdenum sulphide and silicon dioxide powders. A disadvantage of this material was its relatively low compressive strength before failure (about 138 megapascals). Tools we made from Ren TDT-177 also tended to break at the edges. The tooling material also seemed to have a low tolerance to bending and sliding of sheet steel along the working surface of the tool.
As none of the known compositions proved to be rugged enough to stamp automotive body sheet steel, even on a limited production of a few parts, it was our object to develop a novel and improved epoxy-based tooling composition.